1. Field of the Invention
This inventions relates to filtering devices and, more particularly, to precision wound tubular filters with helically wound layers of yarn, roving, or other windable filter material.
2. Description of the Prior Art
The prior art discloses a wide variety of tubular filter elements with rigid pervious cores on which is wound a plurality of layers of filter media which is generally strands of yarn or roving wound on the core in a specified criss-cross or "diamond" pattern. The diamond-shaped filter media pattern creates passages through the filter media through which the fluid to be filtered flows and in which unwanted contaminating particles are trapped. Often, the fibrous strands of yarn or roving which make up the filter media are napped to enhance their contaminant holding characteristics. U.S. Pat. No. 2,368,216 to Hastings et al., U.S. Pat. No. 3,319,793 to Miller et al., and U.S. Pat. No. 3,624,779 to Miller et al. teach various types of these tubular filter elements. U.S. Pat. No. 3,356,226 to Miller et al., U.S. Pat. Nos. 3,398,904 and 3,398,905 to Adams et al., and U.S. Pat. No. 4,253,228 to Easley teach winding apparatus for applying strands of yarn or roving filter media to cores. Filters such as these are subject to an unwanted excessive pressure drop across the filter which is the result of the outer portions of the filter being blocked by filtered contaminant particles. Usually, the fluid to be filtered flows from the outside to the inside of the filter and then through the filter core and out of the filtering device.
U.S. Pat. No. 3,471,028 to Miller et al. discloses the use of two different filter media on a single core, helically wound yarn on the filter's outer portion and wrapped sheets of filter media on the inner portion. United Kingdom patent application 186,358, filed on Sept. 11, 1980, by Womer discloses a precision helically wound filter cartridge that has a base wind and an outer wind in which the number of diamonds on the surface of each of the winds differs from the number of diamonds on the surface of each of the other winds. This difference is created during the winding by altering the diamond pattern so that the number of circumferential diamonds is decreased or increased and the number of axial diamonds is decreased or increased or a combination of both of these procedures is employed.
Generally, in making a wound tubular filter the core is mounted horizontally on a spindle of a winding machine. The spindle is rotated as a strand of yarn or roving is wound on to the core by the winding machine. Appropriate mechanisms are employed to move the guide which guides the strand back and forth along the length of the spindle as it is rotated so that the strand is applied to the core in a precise pattern. The strand is spirally wrapped around the core one end to the other, e.g., from left to right, as the core rotates and then when the strand reaches the right end of the core the strand is spirally wrapped around the core in the opposite direction, i.e., from right to left. Since the core always rotates in the same direction on the spindle, the strands spirally cross each other and form a diamond pattern.
During winding as additional layers of filter media are wound on to the core in a diamond pattern the diameter of the filter increases and, although the individual diamonds change in size, the number of diamonds as counted circumferentially around the cartridge remains the same. The diamond height stays the same since the length of the filter stays the same, but the length of the diamonds increases as the circumference of the filter increases. Consequently, there is an increase in the area of each diamond.
As shown in FIG. 2 the diamonds in a prior art filter increase in size about the circumference of the filter. Also the diamonds are displaced symetrically so that the diamond centers are not in line radially going out from the core but are located on a curve. FIG. 2 is a cross-sectional view of a wound filter taken perpendicularly to the filter's longitudinal axis. Since there is no straight line through the diamonds to the core, the fluid to be filtered must follow the more circuitous path along the diamond curve thereby increasing the dirt holding capacity of the filter.
As taught by U.S. Pat. No. 3,648,846 to Sicard, the area of the diamond can become too large thereby permitting the flow-through of unwanted contaminant particles to the more interior layers of the filter. This patent proposed a solution to the problem by changing the winding to increase the circumferential number of diamonds in successive winding layers. This procedure is inefficient, time-consuming and labor-expensive.
In an effort to provide a wider range of filtering characteristics, attempts have been made to alter the rate of density change from the interior to the outside filter media wound on a core. These attempts have included actually stopping the winding machine during the winding of a filter to change the winding pattern; this has been accomplished by changing the gearing on the winding machine to alter the wind pattern. Other attempts to provide a stepped "stepped" change in the density of a filter have included the provision of extremely complex machines with independently driven double stepped motors. All of these attempts have either been inefficient, exorbitantly expensive, or both.